The invention is directed to optimization of the usage of radio resources in mobile telecommunication systems, particularly during the call set up stage.
The capacity of the air interface is a critical property in any mobile telecommunication system. Therefore, it is desirable that the air interface resources are reserved only when they are needed for transporting user traffic. In the GSM system, this desire gives rise to various call set up strategies, which try to find a compromise between minimizing the use of air interface resources and user convenience. Similar considerations affect also other mobile telecommunications systems such as the UMTS system (Universal Mobile Telephone System), but for simplicity, the following examples of prior art methods focus on the GSM system.
One strategy is the so called very early assignment strategy, in which a full TCH/F channel is allocated at once, when a mobile station requests for a connection. This has the advantage, that call set up time will be rather short, since signaling needed to complete the call set up can be performed using the full capacity of a TCH/F channel.
A second strategy is the so called early assignment strategy, in which a TCH/8 channel is allocated at once, when a mobile station requests for a connection, and a full TCH/F channel is allocated later, when during the signaling the network finds out, that a full TCH/F channel will be required. This strategy saves resources during the call set up, since a TCH/8 channel uses less resources than a TCH/F channel. The drawback is the lengthening of call set up time, since the set up signaling takes a longer time using a TCH/8 channel than using a TCH/F channel.
In a third strategy, named the Off Air Call Set Up (OACSU) procedure, a full TCH/F channel is allocated very late during the call set up, i.e. when the other end answers the call. In this strategy, very little resources are used before the transmission actually commences, e.g. when the subscriber answers. The OACSU procedure is the most resource saving of these three strategies. Nevertheless, it is not widely used due to several drawbacks of the strategy. For example, if allocation of a channel does not succeed immediately when the other end answers the call, the situation is very disturbing for the answering party, who answered a call only to hear complete silence or an automated message until the channel allocation succeeds. Secondly, during call set up inband announcements of various kinds of information are used to assist in the interfacing of different systems to the GSM system. The OACSU procedure and other call set up procedures used in the GSM system are described in further detail for example in the book xe2x80x9cThe GSM System for Mobile Communicationsxe2x80x9d by Michel Mouly and Marie-Bernadette Pautet, ISBN 2-9507190-0-7, Palaiseau 1992.
In cellular telecommunication systems a single speech connection or data connection through the cellular telecommunication network is called a bearer. Generally, a bearer is associated with a set of parameters pertaining to data communication between a certain terminal equipment and a network element, such as a base station or an interworking unit (TWU) connecting the cellular network to another telecommunications network. The set of parameters associated with a bearer comprises typically for example data transmission speed, allowed delays, allowed bit error rate (BER), and the minimum and maximum values for these parameters. One mobile communication means (ME, Mobile Equipment or MS, Mobile Station) may in some cellular telecommunication systems support one bearer only, in some other systems also more than one simultaneous bearers. This specification uses the term bearer in the following instead of the term channel, since the term bearer better reflects the manifold properties of a connection.
An object of the invention is to provide a bearer allocation method, which allows late allocation of resources without increasing the risk of not obtaining the desired radio resources.
The objects are reached by using a two-phase bearer allocation mechanism, in which the bearer is only reserved early in the call set up process, and allocated later, when the called party answers or the transmission can otherwise be observed to begin.
The method according to the invention is characterized by that, which is specified in the characterizing part of the independent method claim. The dependent claims describe further advantageous embodiments of the invention.